The Vancouver Aquarium says a toxin was the cause of death of two beluga whales last November, but the exact substance couldn’t be identified.

The conclusion is the result of a five-month investigation that also determined the toxin was likely introduced by food, water or through human interference.

The aquarium’s head veterinarian, Dr. Martin Haulena, says the investigation has helped them understand what happened and how they can help ensure the safety of the mammals in their care.

The aquarium says that is has taken several steps to protect the remaining animals at the aquarium including enhanced food screening, overhauling its water treatment systems and real-time testing of the circulating water.

The aquarium says in a news release it is also significantly updating its security to monitor perimeter access and reduce potential threats of human interference.

The belugas, 21-year-old Qila and her mother, 30-year-old Aurora, died nine days apart last November from what officials said was a mysterious illness.